The present invention relates to a milling cutter for dressing resistance welding electrodes.
It is known that, in spot-welding plants, the electrodes of the welding guns wear, become contaminated and deform with use; the correct shape and clean state of the electrodes therefore has to be re-established frequently by dressing means with milling cutters. Dressing of the electrodes has improved the performance of production plants since electrode life has been lengthened; the geometric shape and quality of the spot-weld obtained has been regularized and improved.
As is known, particularly in the car-manufacturing field, resistance spot-welds are performed by welding guns that are mounted on robotized arms. Each gun has two opposable electrodes of generally convex and cylindrical external shape. The electrodes wear and are deformed with use. Typically, a crater forms on the front face or “lens” of the electrode, resulting in the formation of an air pocket which reduces electrical conductivity and prevents correct geometrical contact between the electrode and the sheet metal to be welded. Welds performed with an electrode in these conditions are of poor quality, have an annular or asymmetric imprint, and are weak. hi other cases, the end of the electrode adopts a mushroom shape due to zinc deposits and deformation. When an electrode thus deformed is moved away from the sheet metal that has just been welded, it leaves an imprint the edge of which bears visible and undesirable signs of “tearing”. Moreover, the enlargement of the free end of the electrode leads to a reduction in current density and in the pressure exerted by the welding gun. An increase in diameter of from 6 to 7 mm corresponds to a 36% increase in contact surface area (from 26 to 38 mm2). This means that, in some cases, a given current supplied to the welding gun will not permit correct fusion of the sheet metal because of the enlargement of the contact surface and the consequent reduction in current density.
Poor quality of the electrode material inevitably leads to welds of inadequate quality. The electrodes are therefore dressed periodically to re-establish their correct shape; when they are worn out they have to be replaced. To dress the electrodes, the welding robot periodically brings its gun into the vicinity of a dressing head that is provided with its own electric or pneumatic motor which sets a biconcave milling cutter in rotation, and introduces the electrodes into the opposed recesses.
A biconcave milling cutter for resistance-welding electrodes comprising a plurality of blades projecting radially outwards from a longitudinal central axis of the milling cutter is known from U.S. Pat. No. 4,762,446. The blades have cutting edges that are spaced angularly at regular intervals about the axis, with flat rear faces and cooperate, during rotation about the above-mentioned axis, so as to define two opposed, domed recesses each suitable for receiving the end of a respective electrode of a welding gun.
Milling cutters of the above-mentioned type have a disadvantage owing to the fact that they tend to remove variable amounts of material from the electrode, according to the hardness of the electrode and the pressure with which the welding gun presses the electrodes into the milling cutter. Too much copper is removed from the electrode when the electrode is quite soft. Pressure transmitted by the welding gun causes the cutting edges of the milling cutter to penetrate too far into the electrode and, in some cases, the milling cutter jams.